In the mammalian central nervous system (CNS), the transmission of nerve impulses is controlled by the interaction between a neurotransmitter released by a sending neuron and a surface receptor on a receiving neuron, causing excitation of this receiving neuron. L-Glutamate, the most abundant neurotransmitter in the CNS, mediates the major excitatory pathway in mammals, and is referred to as an excitatory amino acid (EAA). The receptors that respond to glutamate are called excitatory amino acid receptors (EAA receptors). See Watkins & Evans, Annual Reviews in Pharmacology and Toxicology, 21: 165 (1981), Monaghan, Bridges, and Cotman, Annual Reviews in Pharmacology and Toxicology, 29: 365 (1989); Watkins, Krogsgaard-Larsen, and Honore, Transactions in Pharmaceutical Science, 11: 25 (1990).
Excitatory amino acid receptors are classified into two general types. Receptors that are directly coupled to the opening of cation channels in the cell membrane of the neurons are termed “ionotropic.” This type of receptor has been subdivided into at least three classes, which are defined by the depolarizing actions of the selective agonists N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainic acid (KA). Five kainate receptors, classified as either high affinity (KA1 and KA2) or low affinity (GluR5, GluR6 and GluR7) kainate receptors have been identified. (Bleakman et al, Molecular Pharmacology, 1996, Vol. 49, No. 4, pp. 581-585).
The second general type of receptor is the G-protein or second messenger-linked “metabotropic” excitatory amino acid receptor. This second type is a highly heterogeneous family of glutamate receptors that are linked to multiple second messenger systems. Based on their amino acid sequence homology, agonist pharmacology, and coupling to transduction mechanisms, the 8 presently known mGluR sub-types are classified into three groups. Group I receptors (mGluR1 and mGluR5) have been shown to be coupled to stimulation of phospholipase C resulting in phosphoinositide hydrolysis and elevation of intracellular Ca++ levels, and, in some expression systems, to modulation of ion channels, such as K+ channels, Ca+ channels, non-selective cation channels, or NMDA receptors. Group II receptors (mGluR2 and mGluR3) and Group III receptors (mGluRs4, 6, 7, and 8) are negatively coupled to adenylcyclase and have been shown to couple to inhibition of cAMP formation when heterologously expressed in mammalian cells, and to G-protein-activated inward rectifying potassium channels in Xenopus oocytes and in unipolar brush cells in the cerebellum. Besides mGluR6, which is essentially only expressed in the retina, the mGluR5 are felt to be widely expressed throughout the central nervous system.
Both types of receptors appear not only to mediate normal synaptic transmission along excitatory pathways, but also participate in the modification of synaptic connections during development and throughout life. Schoepp, Bockaert, and Sladeczek, Trends in Pharmacological Science, 11: 508 (1990); McDonald and Johnson, Brain Research Reviews, 15: 41 (1990).
The excessive or inappropriate stimulation of excitatory amino acid receptors leads to neuronal cell damage or loss by way of a mechanism known as excitotoxicity. This process has been suggested to mediate neuronal degeneration in a variety of conditions. Agonists and antagonists of these receptors may be useful for the treatment of acute and chronic neurodegenerative conditions.